5'-Nucleostidase and 5'-nucleotide phosphodiesterase are enzymes from intestinal mucosa which have many characteristics in common with the well-known alkaline phosphatase from the same source. Our present studies are elucidating these similarities with respect to overall physiochemistry and reaction mechanism. As these are completed, we will extend the comparison of these enzymes to their basic structures. We already have obtained a radioisotope-labelled peptide from the active site of 5'-nucleotide phosphodiesterase and are attempting to determine its amino acid sequence. We are purifying 5'-nucleotidase, so that similar studies may be done on it and on alkaline phosphatase, which is already available. Other enzymes from intestinal mucosa which may be investigated include ATPases and DNases. Alkaline phosphatase and 5'-nucleotide phosphodiesterase, and probably the other phosphatases mentioned, are so similar in many ways that they appear to constitute a "family" of enzymes that differ mainly in their substrate specificity, and may be considered to be descendants of a common precursor in either the evolutionary or developmental sense. We wish to establish not only the degree of structural similarity between members of this "family", but also the structural basis for their differences in substrate specificity.